The role of the lymphatic circulation in transplantation is complex. Properly functioning lymphatics promote allograft health by clearing inflammatory infiltrates and edema fluid but also lead to accelerated rejection by facilitating alloimmune trafficking to the draining lymph nodes. In the first two grant periods of this R01, we have demonstrated how preserving blood microvessels may be a clinically-relevant strategy for preventing chronic lung transplant rejection. How the health of the accompanying lymphatics might influence (positively or negatively) the transplant's fate is still unknown. The purpose of this R01 competitive-renewal application is to apply knowledge gleaned from new lymphedema and microvascular research to study the lymphatic contribution to the overall allograft health. In lung transplantation surgery, the lymphatics are severed and not surgically-reconnected. Preliminary studies indicate that lymphangiogenesis, the growth of new lymphatics from the existing ones, drives the reconnection of the donor and recipient lymphatic vessels and help the lymphatic vessels to regain their drainage function. This ?early lymphangiogenesis? event is harmful to transplants by promoting acute rejection through enhanced alloimmune trafficking, especially when immune danger signals are high following organ implantation. However, a ?late lymphangiogenesis? response, occurring after immune priming when perioperative inflammation has subsided, can be beneficial by efficiently resolving inflammation, restoring fluid balance and inducing immune tolerance. Importantly, pilot results indicate that interfering with the ?early lymphangiogenesis? attenuates alloimmune responses, while inhibiting ?late lymphangiogenesis? accelerates rejection. Emerging evidence suggests that exogenously promoting ?late lymphangiogenesis? (so-called `therapeutic lymphangiogenesis') alleviates lung allograft rejection by attenuating inflammation and edema. Hypoxia, VEGF-C/VEGFR3, Notch, and sphingosine-1-phosphate are the critical mediators of lymphangiogenesis. Understanding how these mediators govern the function of lymphatic circulation can help advance the concept of therapeutic lymphangiogenesis in transplantation. The global hypothesis of this grant is that the biphasic contribution of the transplant lymphatic circulation, to both airway disease and health, is governed by key lymphangiogenic pathways. Specific Aim 1 utilizes pharmacotherapy as well as lymphatic- specific transgenic mice to investigate the concept that transplant lymphatics biphasically participate in both disease pathogenesis and resolution. This aim investigates the roles of VEGF-C/VEGFR3, Notch, S1P and hypoxia-related signaling pathways unique to the negative and positive properties of the lymphatic circulation in airway transplants. Specific Aim 2 uses a series of adoptive cell transfer or deletion experiments and tolerizing immunotherapy to test the hypothesis that the lymphatic circulation is a unique target of rejection that can be conserved to promote tolerance.